1. Field of the Invention
This invention relates to the field of portable communication systems and specifically to a data transceiver circuit which can adaptively alter the receiver limiter operating bias level, based on the detection of data sync word.
2. Description of the Prior Art
Portable radio transceivers are used in several facets of communications technology. Portable data transceivers can be found in paging systems, and two-way communication systems such as those used by police and other public servants. Recently, portable radio transceivers have found use in portable data terminals which are used to communicate with a host computer. Portable data terminals of this type can provide on site computer dianostics of a second host computer.
Most devices which are used for data communications are operated in a synchronous mode. That is, the local clock signal used to operate the portable data terminal is phase compensated so the transitions of the local clock signal are aligned with the transitions of an incoming data signal. In most portable data receiver applications, a data receiver output signal is coupled to a data limiter circuit, which provides binary 1-0 information to a data decoder.
In portable data receivers of this type, a difference in the crystal frequencies of the receiver and transmitter in a radio system will cause an offset voltage to be superimposed on the received signal. The offset voltage may prevent the receiver limiter circuit from switching at the proper threshold point. In the limiter stage, the bias level may be adjusted to compensate for the offset voltage to maintain maximum duty cycle. However, once a word sync signal has been detected, the bias level must not vary when the binary message signal contains long strings of consecutive "ones" or "zeros". The above-mentioned problems are compounded when several portable data transceivers are used in a system. Since the crystal frequencies of each portable unit are generated independently, each portable unit will exhibit unique offset voltage characteristics.
Several prior art techniques have addressed the offset voltage problem. One technique attempts to correct all DC level disturbances in received data so that a fixed decision level can be utilized in the limiter circuit.
Another technique attempts to correct offset voltage disturbances by adjusting the receiver local oscillator frequency until the received signal and the local oscillator are in lock. The resulting circuitry for both of these techniques is quite complex and may be undesirable in a portable unit with limited space and power source capabilities.